1. Field of the Invention
This invention relates to a process for separating a mixture of finely divided minerals into constituents thereof. In particular, the invention relates to a procedure for effecting such separation by a selective flocculation technique wherein both a high molecular weight polymer and a low molecular weight polymer are added to an aqueous slurry containing the mixture of minerals to produce a high yield product which is low in impurities. This invention is especially adapted to the separation of colored impurities, such as titania (anatase and rutile) and micas, from kaolin clay.
2. Description of Related Art
When particles of mineral ore or powder mixtures are sufficiently large, for example, larger than 325 (U.S.) mesh, the components of the mixture can be separated by simple physical means such as air or magnetic separation. When particles are finer, more sophisticated technology may be needed to bring about efficient separations. It is conventional to make the separation of finely divided mineral, e.g., particles finer than 325 mesh, by forming the mixture into an aqueous pulp or slurry and adding chemicals that will bring about a desired separation. One widely used method is froth flotation. In the case of froth flotation of phosphate or oxidized minerals from siliceous gangue, it is conventional to use a fatty acid collector and a salt promoter.
The fatty acid collector coats the mineral particles, which are separated from the siliceous gangue in the form of a froth, and a clay dispersant may be used to keep the clay particles dispersed during the froth flotation process. In the case of kaolin clays which contain significant amount of sub-micron contaminants, conventional froth flotation techniques may not produce the desired removal of colored bodies.
Selective flocculation is a procedure that is widely used commercially to separate finely divided minerals and powders. In the case of clay, some processes utilize anionic polymers to selectively flocculate the clay, leaving the impurities, such as titanium, dispersed and amenable to subsequent separation from the clay. Commercial variants of selective flocculation employ weakly anionic polymers such as hydrolyzed polyacrylamide to selectively flocculate impurities in the clay, leaving the purified clay dispersed. See, for example, U.S. Pat. No. 3,837,482 to Sheridan; U.S. Pat. No. 3,701,417 and 3,862,027, both to Mercade; U.S. Pat. No. 3,371,988 to Maynard, et al.; and U.S. Pat. No. 4,604,369 (Shi).
A variant of the selective flocculation process is where the impurities in the clay are flocculated and the clay remains dispersed in the slurry. Examples of this process are disclosed in U.S. Pat. No. 3,857,781 assigned to Huber; U.S. Pat. No. 4,604,369 assigned to Thiele; and U.S. Pat. Nos. 5,535,890 and 5,603,411 assigned to Engelhard. These processes may employ a high molecular weight organic anionic polymer, such as a water soluble polyacrylamide in excess of 1 million, or a co-polymer of acrylamide, or a copolymer of polyacrylamide and polyacrylate with 95% anionic charge and a molecular weight in excess of 5 million.
U.S. Pat. No. 4,604,369 further teaches the use of ammonium chloride as the conditioning agent and the blunging, diluting, and flocculating steps being done in a substantially continuous manner. U.S. Pat. No. 5,535,890 further teaches the use of a fatty acid and calcium chloride as the conditioning agents. U.S. Pat. No. 5,603,411 uses a mixture of dispersants including soda ash to disperse a slimed ore pulp prior to conditioning with a fatty acid, salt, and then a polymeric flocculating agent.
To achieve selective adsorption of a flocculating agent on a particular component of a mixture, a number of methods have been suggested in the literature [Yu and Attia; in "Flocculation in Biotechnology and Separation Systems," (Y. A. Attia, ed.) p. 601, Elsevier, Amsterdam 1987; Behl, S. and Moudgil, B. M., Minerals and Metallurgical Processing, 5, 92,1992 and, Behl, S. and Moudgil, B. M., Journal of Colloidal Interface Science, 160, 1993]. One of the methods involves selectively blocking the active sites on the inert or nonflocculating component for adsorption of the polymeric flocculating agent. This may be achieved by adsorption of a lower molecular weight fraction of the flocculating agent, which can act as a dispersant and/or site blocking agent prior to exposing the particle surfaces to the flocculating agent.
The selective flocculation processes employing high molecular weight anionic polymers generally suffer from the problem of low product yields due to the high molecular weight polymers attaching onto and settling with the oppositely charged cationic edges of the kaolin clay. Other disadvantages of the processes of the prior art involve a product with a relatively high degree of TiO.sub.2 level such as greater than 0.50% and as high as 1.07%. Additional disadvantages are the formation of small flocs that require a longer settling time (with a larger thickener to maintain production) and minimum shear imparted on the slurry to allow settling. In addition, the presence of fatty acids because of their hydrophobicity may cause performance problems if present in the final product.